Method and apparatus for low complexity frequency synchronization in LTE wireless communication systems

ABSTRACT

In a wireless communication system, a client terminal may first establish time and frequency synchronization with the network. While establishing the time and frequency synchronization, a client terminal may need to detect additional parameters about the network, such as physical cell identity, before it can initiate communication with the wireless communication system. Detecting the network parameters in presence of time and frequency offsets increases the complexity of the initial cell search procedure that includes time and frequency synchronization as well as detection of network parameters. A method and apparatus are disclosed that achieve frequency synchronization earlier in the cell search procedure, which in turn reduces the complexity and improves the performance of the latter stages of cell search procedure and the overall performance of the client terminal.

BACKGROUND

Typically, as shown in FIG. 1, a wireless communication system 10comprises elements such as client terminal or mobile station 12 and basestations 14. Other network devices which may be employed, such as amobile switching center, are not shown. In some wireless communicationsystems there may be only one base station and many client terminalswhile in some other communication systems such as cellular wirelesscommunication systems there are multiple base stations and a largenumber of client terminals communicating with each base station.

As illustrated, the communication path from the base station (BS) to theclient terminal direction is referred to herein as the downlink (DL) andthe communication path from the client terminal to the base stationdirection is referred to herein as the uplink (UL). In some wirelesscommunication systems the client terminal or mobile station (MS)communicates with the BS in both DL and UL directions. For instance,this is the case in cellular telephone systems. In other wirelesscommunication systems the client terminal communicates with the basestations in only one direction, usually the DL. This may occur inapplications such as paging.

The base station with which the client terminal is communicating isreferred to as the serving base station. In some wireless communicationsystems the serving base station is normally referred to as the servingcell. While in practice a cell may include one or more base stations, adistinction is not made between a base station and a cell, and suchterms may be used interchangeably herein. The base stations that are inthe vicinity of the serving base station are called neighbor cell basestations. Similarly, in some wireless communication systems a neighborbase station is normally referred to as a neighbor cell.

Duplexing refers to the ability to provide bidirectional communicationin a system, i.e., from base station to client terminals (DL) and fromclient terminals to base station (UL). There are different methods forproviding bidirectional communication. One commonly used duplexingmethod is Frequency Division Duplexing (FDD). In FDD wirelesscommunication systems, two different frequencies, one for DL and anotherfor UL are used for communication. In a FDD wireless communicationsystem, the client terminals may be receiving and transmittingsimultaneously.

Another commonly used method is Time Division Duplexing (TDD). In TDDbased wireless communication systems, the same exact frequency is usedfor communication in both DL and UL. In TDD wireless communicationsystems, the client terminals may be either receiving or transmittingbut not both simultaneously. The use of the Radio Frequency (RF) channelfor DL and UL may alternate on periodic basis. For example, in every 5ms time duration, during the first half, the RF channel may be used forDL and during the second half, the RF channel may be used for UL. Insome communication systems the time duration for which the RF channel isused for DL and UL may be adjustable and may be changed dynamically.

Yet another commonly used duplexing method is Half-duplex FDD (H-FDD).In this method, different frequencies are used for DL and UL but theclient terminals may not perform receive and transmit operations at thesame time. Similar to TDD wireless communication systems, a clientterminal using the H-FDD method must periodically switch between DL andUL operation. All three duplexing methods are illustrated in FIG. 2.

In many wireless communication systems, normally the communicationbetween the base station and client terminals is organized into framesas shown in FIG. 3. The frame duration may be different for differentcommunication systems and normally it may be in the order ofmilliseconds. For a given communication system the frame duration may befixed. For example, the frame duration may be 10 milliseconds.

In a TDD wireless communication system, a frame may be divided into a DLsubframe and a UL subframe. In TDD wireless communication systems, thecommunication from base station to the client terminal (DL) directiontakes place during the DL subframe and the communication from clientterminal to network (UL) direction takes place during UL subframe on thesame RF channel.

Orthogonal Frequency Division Multiplexing (OFDM) systems typically useCyclic Prefix (CP) to combat inter-symbol interference and to maintainthe subcarriers orthogonal to each other under multipath fadingpropagation environment. The CP is a portion of the sample data that iscopied from the tail part of an OFDM symbol to the beginning of the OFDMsymbol as shown in FIG. 4. One or more OFDM symbols in sequence as shownin FIG. 4 are referred herein as an OFDM signal.

In addition to the purposes mentioned above, the CP is often used forfrequency offset estimation at the receiver. Any frequency offset at thereceiver relative to the center frequency of the transmitted signalcauses the phase of the received signal to change linearly as a functionof time. The two parts of an OFDM signal that are identical at thetransmitter, i.e., the CP and the tail portion of the OFDM symbol, mayundergo different phase change at the receiver due to the frequencyoffset. Therefore, the frequency offset can be estimated by performingcorrelation between the CP and the tail portion of an OFDM symbol. Theangle of the CP correlation indicates the amount of phase rotation thatis accumulated over the duration of an OFDM symbol. This accumulatedphase rotation may then used for frequency offset estimation.

The frequency offset at the receiver during initial synchronization maybe very high. Furthermore, since the client terminal may not besynchronized to any base station during initial synchronization, theOFDM symbol boundaries are not known to the client terminal. In wirelesscommunication system deployments where frequency reuse is employed, thesignals from several base stations may be superimposed. In some cases,the various base stations may not be time synchronized, i.e., the OFDMsymbol boundaries for the different cells may not be time aligned. Evenif the OFDM symbol boundaries are time aligned at the base stations, thepropagation delays from different base stations to the client terminalmay be different and therefore the OFDM symbol timing may not be timealigned at the client terminal receiver. Furthermore, in some wirelesscommunication systems, such as 3^(rd) Generation Partnership Project(3GPP) Long Term Evolution (LTE) or LTE-Advanced wireless communicationsystems, an option of using different CP lengths exists and the exact CPin use may not be known a priori to the client terminal. In addition,the different base stations whose signals may be superimposed may beusing different CP length. The overall received signal scenario isillustrated in FIG. 5. In case of TDD systems since the same frequencyis used for transmit and receive, at power up the client terminal maynot be aware of the boundary between DL and UL. In case of TDD systems,the significant power difference between DL and UL may create challengesin performing frequency offset estimation and may lead to inaccuratefrequency offset.

Most wireless communication systems may employ some form of framing inthe air interface. For example, 10 ms radio frames are used in the 3GPPLTE wireless communication systems and each radio frame comprises 10subframes as shown in FIG. 6. Each subframe in turn consists of twoslots and each slot consists of 6 or 7 OFDM symbols depending on thetype of CP used as shown in FIG. 6. In the 3GPP LTE wirelesscommunication system, two different CP lengths are used and they arereferred to as Normal CP and Extended CP. In wireless communicationsystems, normally the specific air interface frame structure repeatsitself over certain periodicity.

The 3GPP LTE wireless communication system uses the followingsynchronization signals to assist the client terminal in achieving timeand frequency synchronization as well as the detection of physical layercell identity:

-   -   Primary Synchronization Signal (PSS)    -   Secondary Synchronization Signal (SSS)        The positions of the PSS and SSS are illustrated in the FIG. 7        for FDD air-interface of a 3GPP LTE wireless communication        system. Note that the figure shows the position of the PSS and        SSS for both the Normal CP and Extended CP. FIG. 8 illustrates        the PSS and SSS positions for TDD air-interface of 3GPP LTE        wireless communication system. The PSS and SSS for different        cells may be different as described below.

The different PSS and SSS are identified by different signal sequencesused for transmission. Specifically, 504 physical cell identities aredefined in 3GPP LTE wireless communication system specifications andthey are organized into 168 groups with three identities in each group.The SSS sequence identifies the physical cell identity group and PSSidentifies the physical cell identity within a group. Detecting aphysical cell identity requires the detection of both the PSS and theSSS.

The PSS sequence in frequency domain is a length 63 Zadoff-Chu sequenceextended with five zeros on each side and mapped to central 72sub-carriers as shown in FIG. 9. The Direct Current (DC) subcarrier isnot used. In 3GPP LTE wireless communication system three different PSSsequences are used with Zadoff-Chu root indices 24, 29 and 34corresponding to cell identity 0, 1 and 2 respectively within thephysical cell identity group. The exact PSS sequences are defined in the3GPP LTE specification TS 36.211 “Physical Channels and Modulation.” Thetime domain PSS signal may be obtained by performing Inverse DiscreteFourier Transform (IDFT) of the frequency domain PSS. The two timedomain PSS instances present within a 10 ms radio frame as shown in FIG.7 and FIG. 8 are identical.

The SSS sequences in frequency domain are generated by frequencyinterlacing of two length-31 M-sequences X and Y, each of which may take31 different M-values. The SSS is extended with five zeros on each sideand mapped to central 72 sub-carriers as shown in FIG. 10. The DCsubcarrier is not used. In 3GPP LTE wireless communication system, 168valid combinations of X and Y are defined corresponding to 168 differentphysical cell identity groups. The time domain SSS signal may beobtained by performing IDFT of the frequency domain SSS. The two SSSsequences present in a 10 ms radio frame are different, namely SSS₁ andSSS₂ as shown in FIG. 7 and FIG. 8, which allows the client terminal todetect 10 ms radio frame timing from reception of a single SSS. The onlydifference between SSS₁ and SSS₂ is that the two M-sequences X and Yused in SSS₁ are swapped in SSS₂. Relative timing between SSS and PSSdepends upon CP type and duplexing type as shown in FIG. 7 and FIG. 8.

Since OFDM symbol synchronization may be achieved at the end of PSSsearch, frequency domain processing may be employed for furtheranalysis, such as SSS detection.

The SSS search has to handle timing and frequency offset ambiguities inaddition to other system unknowns such as CP type and duplexing type.The relative timing (in terms of number of samples) between SSS and PSSvaries depending upon CP and duplexing type. Multiple SSS searchattempts may be required to resolve unknown system parameters such as CPtype and duplexing type. If CP type is known prior to SSS detection, forexample using CP correlator, corresponding SSS detection attempt may beskipped. The PSS detection may result in multiple possible PSS positionsbeing detected due to the presence of multiple cells surrounding theclient terminal.

Frequency offset in OFDM systems generally manifests itself in twocomponents commonly referred as integer frequency offset and fractionalfrequency offset. Integer frequency offset refers to the frequencyoffset in terms of integral number of the subcarriers and the fractionalfrequency offset refers to the frequency offset remaining afterexcluding the integer frequency offset. In 3GPP LTE wirelesscommunication system the frequency spacing between subcarriers is 15kHz. Therefore, for example, a frequency offset of 35 kHz at the clientterminal manifests itself as two subcarrier offset (30 kHz) and afractional frequency offset of 5 kHz.

Fractional frequency offset may be compensated by estimating fractionalfrequency offset using conventional methods such as CP correlation. Inconventional systems, the integer frequency offset may be detected infrequency domain by attempting to decode SSS with different assumptionsabout different SSS frequency bin positions.

The SSS detection requires selection of one out of the 168 possiblevalid combinations. When coupled with additional unknowns such as theinteger frequency offset, the CP type, the duplexing type, the number ofsearch candidates for SSS becomes excessive leading to high complexityand high power consumption. To handle integer frequency offset of ±30kHz (two subcarrier spacing), the number of SSS frequency domainprocessing iterations required is five corresponding to the nominalposition and two subcarrier offsets in both positive and negativedirections. When the CP and duplexing type are not known, the number ofcombinations goes up to 20.

SUMMARY

A method and apparatus are disclosed that may achieve the integerfrequency offset estimation and compensation before the start of SSSsearch. This in turn may reduce the complexity of the SSS detection aswell. A fake cell detection is defined herein as successful SSSdetection for a signal when there is no real cell present at thedetected SSS position. A detected fake cell may be reported by theclient terminal to the network. The network in turn may do a handover ofthe client terminal to the reported cell and since it is not a real cellthe handover may fail leading to reduced performance. The disclosedmethod may also reduce the potential fake cells that may be detectedwhen SSS detection is attempted over a large number of possiblecandidates corresponding to different integer frequency offsets.

In accordance with an aspect of the present invention, a method forfrequency synchronization of a signal may include controlling, by aprocessing device: extracting, from time domain samples of the signal,samples corresponding to a strongest detected Primary SynchronizationSignal (PSS); compensating the extracted samples for a fractionalfrequency offset detected from the time domain samples, by applying aphase rotation corresponding to a negative of the fractional frequencyoffset; converting the compensated extracted samples to frequency domainsamples; performing frequency domain even symmetry cross-correlation ofthe frequency domain samples, over a predetermined number of frequencybin positions, to obtain frequency domain PSS even symmetrycross-correlations; and determining a frequency bin position of thefrequency bin positions corresponding to a frequency domain PSS evensymmetry cross-correlation of the frequency domain PSS even symmetrycross-correlations having a maximum magnitude, wherein the determinedfrequency bin position indicates an integer frequency offset in thesignal.

In one alternative, the frequency domain even symmetry cross-correlationmay be performed over the predetermined number of frequency binpositions around a nominal position of the PSS.

In one alternative, the method may include controlling, by theprocessing device: determining a frequency offset of the signal bycombining the integer frequency offset indicated by the determinedfrequency bin position with the fractional frequency offset, andsearching the signal compensated by the frequency offset for a SecondarySynchronization Signal (SSS).

In one alternative, the method may include controlling, by theprocessing device, SSS processing of the signal compensated by thefrequency offset.

In one alternative, the strongest detected PSS may correspond to astrongest PSS time domain correlation.

In one alternative, the strongest PSS time domain correlation may bedetermined from a magnitude of a time domain cross-correlation of thePSS.

In accordance with an aspect of the present invention, an apparatus forfrequency synchronization of a signal may include circuitry configuredto control: extracting, from time domain samples of the signal, samplescorresponding to a strongest detected Primary Synchronization Signal(PSS); compensating the extracted samples for a fractional frequencyoffset detected from the time domain samples, by applying a phaserotation corresponding to a negative of the fractional frequency offset;converting the compensated extracted samples to frequency domainsamples; performing frequency domain even symmetry cross-correlation ofthe frequency domain samples, over a predetermined number of frequencybin positions, to obtain frequency domain PSS even symmetrycross-correlations; and determining a frequency bin position of thefrequency bin positions corresponding to a frequency domain PSS evensymmetry cross-correlation of the frequency domain PSS even symmetrycross-correlations having a maximum magnitude, wherein the determinedfrequency bin position indicates an integer frequency offset in thesignal.

In one alternative of the apparatus, the frequency domain even symmetrycross-correlation may be performed over the predetermined number offrequency bin positions around a nominal position of the PSS.

In one alternative of the apparatus, the circuitry may be configured tocontrol: determining a frequency offset of the signal by combining theinteger frequency offset indicated by the determined frequency binposition with the fractional frequency offset, and searching the signalcompensated by the frequency offset for a Secondary SynchronizationSignal (SSS).

In one alternative of the apparatus, the circuitry may be configured tocontrol SSS processing of the signal compensated by the frequencyoffset.

In one alternative of the apparatus, the strongest detected PSS maycorrespond to a strongest PSS time domain correlation.

In one alternative of the apparatus, the strongest PSS time domaincorrelation may be determined from a magnitude of a time domaincross-correlation of the PSS.

In accordance with an aspect of the present invention, a wirelesscommunication device may include a receiver to receive a signal and aprocessing device. The processing device may be configured to controlfrequency synchronization of a signal, by controlling: extracting, fromtime domain samples of the signal, samples corresponding to a strongestdetected Primary Synchronization Signal (PSS); compensating theextracted samples for a fractional frequency offset detected from thetime domain samples, by applying a phase rotation corresponding to anegative of the fractional frequency offset; converting the compensatedextracted samples to frequency domain samples; performing frequencydomain even symmetry cross-correlation of the frequency domain samples,over a predetermined number of frequency bin positions, to obtainfrequency domain PSS even symmetry cross-correlations; and determining afrequency bin position of the frequency bin positions corresponding to afrequency domain PSS even symmetry cross-correlation of the frequencydomain PSS even symmetry cross-correlations having a maximum magnitude,wherein the determined frequency bin position indicates an integerfrequency offset in the signal.

In one alternative of the device, the frequency domain even symmetrycross-correlation may be performed over the predetermined number offrequency bin positions around a nominal position of the PSS.

In one alternative of the device, the processing device may beconfigured to control: determining a frequency offset of the signal bycombining the integer frequency offset indicated by the determinedfrequency bin position with the fractional frequency offset, andsearching the signal compensated by the frequency offset for a SecondarySynchronization Signal (SSS).

In one alternative of the device, the processing device may beconfigured to control SSS processing of the signal compensated by thefrequency offset.

In one alternative of the device, the strongest detected PSS maycorrespond to a strongest PSS time domain correlation.

In one alternative of the device, the strongest PSS time domaincorrelation may be determined from a magnitude of a time domaincross-correlation of the PSS.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventional wireless cellular communicationsystem.

FIG. 2 illustrates FDD, TDD and H-FDD duplexing techniques.

FIG. 3 illustrates a TDD frame consisting of DL portions and ULportions.

FIG. 4 illustrates an OFDM symbol with Cyclic Prefix Insertion.

FIG. 5 illustrates OFDM signals from a set of base stations that are nottime synchronized and using different CP lengths.

FIG. 6 illustrates the frame structure of the air interface of the 3GPPLTE wireless communication system.

FIG. 7 illustrates the location of PSS and SSS for Normal CP andExtended CP in the case of an FDD 3GPP LTE wireless communicationsystem.

FIG. 8 illustrates the location of PSS and SSS for Normal CP andExtended CP in case of TDD 3GPP LTE wireless communication system.

FIG. 9 illustrates PSS generation procedures for 3GPP LTE wirelesscommunication system.

FIG. 10 illustrates SSS generation procedures for 3GPP LTE wirelesscommunication system.

FIG. 11 illustrates the frequency domain PSS signal for root index 29for the 3GPP LTE wireless communication system.

FIG. 12 illustrates the frequency domain even symmetry correlation forPSS signal for the 3GPP LTE wireless communication system.

FIG. 13 illustrates a frequency domain PSS even symmetry correlationmethod for integer frequency offset estimation according to aspects ofthe present invention.

FIG. 14 illustrates an example flow diagram for processing stepsaccording to aspects of the present invention.

FIG. 15 illustrates a PSS frequency domain even symmetry correlation fornominal position according to aspects of the present invention.

FIG. 16 illustrates a PSS frequency domain even symmetry correlation forone subcarrier offset corresponding to −15 KHz frequency offsetaccording to aspects of the present invention.

FIG. 17 illustrates a PSS frequency domain even symmetry correlation forone subcarrier offset corresponding to +15 KHz frequency offsetaccording to aspects of the present invention.

FIG. 18 illustrates a wireless mobile station diagram, which may beemployed with aspects of the invention described herein.

FIG. 19 illustrates an application processor subsystem for a wirelessmobile station, which may be employed with aspects of the inventiondescribed herein.

FIG. 20 illustrates a baseband subsystem for a wireless mobile station,which may be employed with aspects of the invention described herein.

FIG. 21 illustrates an RF subsystem for a wireless mobile station, whichmay be employed with aspects of the invention described herein.

DETAILED DESCRIPTION

The foregoing aspects, features and advantages of the present inventionwill be further appreciated when considered with reference to thefollowing description of exemplary embodiments and accompanyingdrawings, wherein like reference numerals represent like elements. Indescribing the exemplary embodiments of the invention illustrated in theappended drawings, specific terminology will be used for the sake ofclarity. However, the invention is not intended to be limited to thespecific terms used.

According to an aspect of the present invention, the fractionalfrequency offset present in the detected PSS signal is compensated byapplying phase rotation corresponding to the negative of the detectedfractional frequency offset. The PSS detection may be performed usingone of the conventional methods such as cross-correlation of incomingtime domain signal with time domain local replica of all three possiblePSS indices. The fractional frequency offset may be detected using oneof the conventional methods such as CP correlation. According to anaspect of the present invention, the fractional frequency offsetcompensated PSS signal with strongest cross-correlation metric is thenconverted to frequency domain for integer frequency offset estimation.

According to an aspect of the present invention, the even symmetry ofthe PSS signal in frequency domain may be used to determine the integerfrequency offset. FIG. 11 illustrates the 62-point signal for PSS rootindex 29 in frequency domain. It is evident from the figure that thesignal is symmetric around its center for both the real and imaginaryparts. This is further confirmed by the correlation of the first half ofthe signal with the conjugate of the second half. This even symmetrycorrelation is illustrated in FIG. 12 which has a unity peak when thecorrelation is taken by halving the signal exactly in the middle. Thesame even symmetry is present for all three PSS root indices used in the3GPP LTE wireless communication system.

According to the aspect of the present invention, the even symmetrycorrelation is performed on the received frequency domain PSS signal.According to another aspect of the present invention, the even symmetrycorrelation in frequency domain may be performed over a configurablenumber of frequency bin positions around the nominal position of PSS asshown in FIG. 13. According to another aspect of the present invention,the magnitude of the frequency domain even symmetry correlation may becomputed and a maximum may be searched over all frequency bin positionsover which the even symmetry correlation is computed. According toanother aspect of the present invention, the frequency bin correspondingto the maximum even symmetry correlation magnitude may indicate theinteger frequency offset present in the received signal. The detectedinteger frequency offset may be combined with the fractional frequencyoffset to compute the composite frequency offset. The compositefrequency offset may then used to adjust the frequency of the oscillatorused in the client terminal. For example, a Voltage Controlled CrystalOscillator (VCXO), a Temperature Compensated Crystal Oscillator (TCXO),a Temperature Compensated Voltage Controlled Crystal Oscillator(TCVCXO), etc. may be used. According to another aspect of theinvention, the SSS search may be performed over a signal which is fullycompensated for frequency offset. The use of frequency offsetcompensated signal may enable improved SSS detection performance. Thisenables the SSS search to be performed only over the nominal SSSfrequency bin position. This may reduce the complexity of the SSS searchwhich in turn may reduce power consumption and may reduce theprobability of detecting fake cells. The probability of detecting fakecells may be increased when a client terminal performs SSS searchcorresponding to different combinations of possible CP lengths and thedifferent integer frequency offsets.

According to an aspect of the present invention, the frequency offsetmay be removed before the start of SSS search and SSS processing may beperformed on frequency offset compensated signal. The SSS search mayonly need to handle the unknown CP type and duplexing type. According toan aspect of the present invention, even when there may be multipledetected PSS time offsets, the integer frequency offset detection may beperformed only once. According to an aspect of the present invention,the integer frequency offset estimation based on PSS may be performedusing the PSS signal corresponding to the strongest PSS time domaincross-correlation. The strongest PSS time domain correlation may bedetermined by computing the magnitude of the time domaincross-correlation of PSS. The SSS search may be performed on thefrequency offset compensated signal corresponding to all the detectedPSS time offsets of interest. The complexity of integer frequency offsetestimation using PSS even symmetry correlation in frequency domain maybe much less than the SSS frequency domain processing. Therefore,significant processing reduction and power consumption reduction may beachieved when SSS search combinations are reduced.

The flow diagram 1400 contained in FIG. 14 illustrates an exemplaryinteger frequency offset determination method according to variousaspects of the present invention. At processing stage 1402, the incomingtime domain signal is received and stored. Aspects of the presentinvention may be implemented with or without storing the incoming timedomain signal. At processing stage 1404, the fractional frequency offsetestimation is performed. This may be done using conventional techniquessuch as CP correlation. At processing stage 1406, the PSS searchoperation is performed. This may be done using conventional techniquessuch as PSS time domain cross-correlation with local replicas of allthree possible PSS indices. The processing in the processing stages 1404and 1406 may be done in parallel or sequentially. Regardless of themethod used, the processing continues at processing stage 1408 where theprocessing relevant to the present invention begins. At processing stage1408, the PSS root index and timing position relative to the beginningof the PSS search window are determined for the strongest PSS detectionmetric, such as the time domain cross-correlation magnitude. Atprocessing stage 1410, the samples corresponding to the strongest PSSmay be extracted from the stored samples. In an alternative embodiment,if the samples are not stored at processing stage 1402, a new receivewindow may be opened to capture samples of another instance of thedetected strongest PSS. Regardless of the method used, at processingstage 1412, the samples corresponding to the strongest PSS arecompensated for the fractional frequency offset estimated in processingstage 1404. This may be accomplished by performing linear phase rotationcorresponding to the negative of the detected fractional frequencyoffset. At processing stage 1414, the fractional frequency offsetcompensated samples are converted to frequency domain. At processingstage 1416, frequency domain even symmetry correlation is performed onthe frequency domain samples of the received signal. At processing stage1416, the even symmetry correlation is performed over the configurednumber of frequency bin positions. At processing stage 1418, adetermination is made about the strongest frequency domain PSS evensymmetry correlation magnitude and the corresponding frequency binposition. The frequency bin position corresponding to the strongest PSSfrequency domain even symmetry correlation magnitude may be used as anindicator of the integer frequency offset. At processing stage 1420, theestimated fractional and integer frequency offset are combined to form acomposite estimated frequency offset. The processing may terminate atstage 1422. The composite frequency offset may be used to adjust thelocal oscillator frequency to align with the frequency of the wirelesscommunication network.

The details of the frequency domain even symmetry correlation of thereceived frequency domain PSS symbol are shown in FIG. 15. The left setof 31 subcarriers of the central 62 subcarriers from the received signalare multiplied with the conjugate of the right set of 31 subcarriers ona subcarrier by subcarrier basis as shown in FIG. 15. The product of themultiplication from all 31 pairs is summed together. The output of theadder may be a complex number. The output of the adder is given to themagnitude calculator that performs the absolute value computation bytaking the square root of the sum of the square of the real part and theimaginary part of the complex number i.e.,

$\sqrt{{real}^{2} + {imag}^{2}}.$Note that the first half and the second half of the frequency domain PSSsymbol are selected considering the nominal position of the central 62subcarriers when the integer frequency offset may be zero. To computethe frequency domain even symmetry correlation for one frequency binoffset, corresponding to integer frequency offset of −15 kHz (onesubcarrier), the 31-pairs for the even symmetry correlation of thereceived frequency domain signal are formed as shown in FIG. 16 wherethe pairs are formed by using one frequency bin to the left of thecenter. The rest of the even symmetry correlation computations remainthe same as in FIG. 15. To compute the frequency domain even symmetrycorrelation for one frequency bin offset, corresponding to integerfrequency offset of +15 kHz (one subcarrier), the local replica isaligned with the received frequency domain signal as shown in FIG. 17.The rest of the even symmetry correlation computations remain the sameas in FIG. 15. The frequency domain PSS even symmetry correlation may beperformed for as many expected integer frequency offsets as possible upto ±5 frequency bins.

By way of example only, the above-described method may be implemented ina receiver, e.g., a user device such as a wireless mobile station (MS)12 as shown in FIG. 1.

As shown in FIG. 18, MS 100 may include an application processorsubsystem 101, baseband subsystem 102 and a radio frequency (RF)subsystem 104 for use with a wireless communication network. Adisplay/user interface 106 provides information to and receives inputfrom the user. By way of example, the user interface may include one ormore actuators, a speaker and a microphone. In some mobile devices,certain combination of the application processor subsystem 101, thebaseband subsystem 102 and the RF subsystem 104 are all integrated asone integrated chip.

The application processor subsystem 101 as shown in FIG. 19 may includea controller 108 such as a microcontroller, another processor or othercircuitry. The baseband subsystem 102 as shown in FIG. 20 may include acontroller 118 such as a microcontroller or other processor. The RFsubsystem 104 as shown in FIG. 21 may include a controller 128 such as amicrocontroller, another processor or other circuitry. The controller108 desirably handles overall operation of the MS 100. This may be doneby any combination of hardware, software and firmware running on thecontroller 108. Such a combination of hardware, software and firmwaremay embody any methods in accordance with aspects of the presentinvention.

Peripherals 114 such as a full or partial keyboard, video or still imagedisplay, audio interface, etc may be employed and managed through thecontroller 108.

Aspects of the present invention may be implemented in firmware of thecontroller 108 of the application processor and/or the controller 118 ofthe baseband subsystem. In another alternative, aspects of the presentinvention may also be implemented as a combination of firmware andhardware of the application processor subsystem 101 and/or the basebandsubsystem 102. For instance, a signal processing entity of any or all ofthe FIG. 20 may be implemented in firmware, hardware and/or software. Itmay be part of the baseband subsystem, the receiver subsystem or beassociated with both subsystems. In one example, the controller 118and/or the signal processor 110 may include or control the protocolentity circuitry. The software may reside in internal or external memoryand any data may be stored in such memory. The hardware may be anapplication specific integrated circuit (ASIC), field programmable gatearray (FPGA), discrete logic components or any combination of suchdevices. The terms controller and processor are used interchangeablyherein.

The consumer electronics devices that may use the aspects of theinvention may include smartphones, tablets, laptops, gaming consoles,cameras, video camcorders, TV, car entertainment systems, etc.

Although aspects of the invention herein have been described withreference to particular embodiments, it is to be understood that theseembodiments are merely illustrative of the principles and applicationsof the aspects of the present invention. It is therefore to beunderstood that numerous modifications may be made to the illustrativeembodiments and that other arrangements may be devised without departingfrom the spirit and scope of the aspects of the present invention asdefined by the appended claims. Aspects of each embodiment may beemployed in the other embodiments described herein.

The invention claimed is:
 1. A method for frequency synchronization of asignal, the method comprising: controlling, by a processing device:extracting, from time domain samples of the signal, samplescorresponding to a strongest detected Primary Synchronization Signal(PSS); compensating the extracted samples for a fractional frequencyoffset detected from the time domain samples, by applying a phaserotation corresponding to a negative of the fractional frequency offset;converting the compensated extracted samples to frequency domainsamples; detecting an integer frequency offset in the signal by:performing frequency domain even symmetry cross-correlation of thefrequency domain samples, over a predetermined number of frequency binpositions for at least one selected frequency bin offset, to obtainfrequency domain PSS even symmetry cross-correlations, and determining afrequency bin position of the frequency bin positions corresponding to afrequency domain PSS even symmetry cross-correlation of the frequencydomain PSS even symmetry cross-correlations having a maximum magnitude,wherein the determined frequency bin position indicates the integerfrequency offset in the signal; and adjusting an oscillator using theinteger frequency offset.
 2. The method of claim 1, wherein thefrequency domain even symmetry cross-correlation is performed over thepredetermined number of frequency bin positions around a nominalposition of the PSS.
 3. The method of claim 1, further comprising:controlling, by the processing device: determining a frequency offset ofthe signal by combining the integer frequency offset indicated by thedetermined frequency bin position with the fractional frequency offset,and searching the signal compensated by the frequency offset for aSecondary Synchronization Signal (SSS).
 4. The method of claim 3,further comprising: controlling, by the processing device, SSSprocessing of the signal compensated by the frequency offset.
 5. Themethod of claim 1, wherein the strongest detected PSS corresponds to astrongest PSS time domain correlation.
 6. The method of claim 5, whereinthe strongest PSS time domain correlation is determined from a magnitudeof a time domain cross-correlation of the PSS.
 7. An apparatus forfrequency synchronization of a signal, the apparatus, comprising:circuitry configured to control: extracting, from time domain samples ofthe signal, samples corresponding to a strongest detected PrimarySynchronization Signal (PSS); compensating the extracted samples for afractional frequency offset detected from the time domain samples, byapplying a phase rotation corresponding to a negative of the fractionalfrequency offset; converting the compensated extracted samples tofrequency domain samples; detecting an integer frequency offset in thesignal by: performing frequency domain even symmetry cross-correlationof the frequency domain samples, over a predetermined number offrequency bin positions for at least one selected frequency bin offset,to obtain frequency domain PSS even symmetry cross-correlations, anddetermining a frequency bin position of the frequency bin positionscorresponding to a frequency domain PSS even symmetry cross-correlationof the frequency domain PSS even symmetry cross-correlations having amaximum magnitude, wherein the determined frequency bin positionindicates the integer frequency offset in the signal; and adjusting anoscillator using the integer frequency offset.
 8. The apparatus of claim7, wherein the frequency domain even symmetry cross-correlation isperformed over the predetermined number of frequency bin positionsaround a nominal position of the PSS.
 9. The apparatus of claim 7,wherein the circuitry is configured to control: determining a frequencyoffset of the signal by combining the integer frequency offset indicatedby the determined frequency bin position with the fractional frequencyoffset, and searching the signal compensated by the frequency offset fora Secondary Synchronization Signal (SSS).
 10. The apparatus of claim 9,wherein the circuitry is configured to control SSS processing of thesignal compensated by the frequency offset.
 11. The apparatus of claim7, wherein the strongest detected PSS corresponds to a strongest PSStime domain correlation.
 12. The apparatus of claim 11, wherein thestrongest PSS time domain correlation is determined from a magnitude ofa time domain cross-correlation of the PSS.
 13. A wireless communicationdevice comprising: a receiver to receive a signal; and a processingdevice configured to control frequency synchronization of a signal, bycontrolling: extracting, from time domain samples of the signal, samplescorresponding to a strongest detected Primary Synchronization Signal(PSS); compensating the extracted samples for a fractional frequencyoffset detected from the time domain samples, by applying a phaserotation corresponding to a negative of the fractional frequency offset;converting the compensated extracted samples to frequency domainsamples; detecting an integer frequency offset in the signal by:performing frequency domain even symmetry cross-correlation of thefrequency domain samples, over a predetermined number of frequency binpositions for at least one selected frequency bin offset, to obtainfrequency domain PSS even symmetry cross-correlations, and determining afrequency bin position of the frequency bin positions corresponding to afrequency domain PSS even symmetry cross-correlation of the frequencydomain PSS even symmetry cross-correlations having a maximum magnitude,wherein the determined frequency bin position indicates an integerfrequency offset in the signal; and adjusting an oscillator using theinteger frequency offset.
 14. The device of claim 13, wherein thefrequency domain even symmetry cross-correlation is performed over thepredetermined number of frequency bin positions around a nominalposition of the PSS.
 15. The device of claim 13, wherein the processingdevice is configured to control: determining a frequency offset of thesignal by combining the integer frequency offset indicated by thedetermined frequency bin position with the fractional frequency offset,and searching the signal compensated by the frequency offset for aSecondary Synchronization Signal (SSS).
 16. The device of claim 15,wherein the processing device is configured to control SSS processing ofthe signal compensated by the frequency offset.
 17. The device of claim13, wherein the strongest detected PSS corresponds to a strongest PSStime domain correlation.
 18. The device of claim 17, wherein thestrongest PSS time domain correlation is determined from a magnitude ofa time domain cross-correlation of the PSS.